The current threat of anthrax as an agent of biowarfare makes an understanding of the mechanisms used by anthrax toxins to suppress the innate immune response during the infectious process an important challenge for biomedical research. Much of what is currently known about the pathogenesis of anthrax infection derives from studies conducted in animals, primarily mice, where clear differences from human responses are evident. Since little is known about the molecular basis for the pathogenesis of Bacillus anthracis and its toxins in humans, we propose in this Program Project to investigate in molecular detail how Bacillus anthracis interacts with human host defense mechanisms. The individual projects in this Program will probe the molecular targets and biochemical consequences of B. anthracis action in airway epithelial cells, macrophages and neutrophils, and will define genetic elements that determine susceptibility to anthrax infection and progression. Each project will, however, relate to the central theme of investigating innate host defense mechanisms and share commonality in the molecular mechanisms to be studied. Project 1 will search for genetic elements that modulate the organismal response to lethal toxin using a genetic screen in mice. Project 2 will investigate the influence of B. anthracis and its toxins on human alveolar macrophage phagolysosome function and on their ability to generate inflammatory mediators. Project 3 will examine the responses of human airway epithelial cells to B. anthracis and its toxins, including the involvement of Toll receptors, Rho GTPases, and Nox's in cellular responses. Project 4 will elucidate the molecular basis for the action of anthrax toxins to suppress the chemotactic and oxidative responses of human neutrophils and macrophages, and will investigate the basis for differences in toxin action between these types of leukocyte. Project 5 will assess human Nox proteins as components of host defense and evaluate how variations in the genetic background of humans may influence their responsiveness to B. anthracis infection. Two cores are proposed in support of this Program: an Administrative Core and a Microbiological and Biological Reagents Core. This Program Project will leverage the diverse areas of expertise, resources, and experimental approaches of its members into a focused, highly interactive investigation of human innate immune responsiveness during infection by anthrax. Together these projects will characterize key molecular targets and mechanisms that contribute to the virulence of anthrax. Efforts to identify the process leading to innate immune suppression by anthrax will provide a new molecular basis for disease intervention.